This project is directed at extending our knowledge concerning the physiology of deep hypothermia with the objectives of improving its quality and extending the safe duration of circulatory arrest as an adjunct to open heart surgery in infants. The specific work to be done in the coming year includes extending the safe limit of circulatory arrest to 90 minutes by inducing deeper levels of hypothermia with cardiopulmonary bypass as well as with pure surface cooling. Studies of platelet function during hypothermia will be continued. The influence of various anesthetic agents on these parameters during cooling and rewarming will be evaluated as well as the long-range effects. The thermodilution technique of measuring cardiac output during hypothermia will be correlated with a new method of continuous cardiac output determinations utilizing an ultrasound technique as developed by the Department of Anesthesiology in our Institution. This technique will greatly facilitate exploration of the factors which lead to the low cardiac output symdrome occasionally seen postoperatively. Studies of the neuropathologic basis of the high-stepping gait, which now appears to be localized in the pons by means of electronmicroscopy will be continued. The convergence point for this work is to improve our technique for the protection of the infant's organ systems while the mechanical correction of the patient's heart is accomplished. The greatest need at the present time in surgery for congenital heart disease is to be able to perform more extensive and complex procedures on the critically ill and depleted infant.